A bid to overturn the child sex abuse conviction of a former boarding school worker found guilty by a jury following his suicide during his trial has succeeded.

Darren Turk, 54, hanged himself last June while he was on trial accused of offences against boys aged between 11 and 15 at Frewen College in Northiam, East Sussex, between 1 996 and 2002.

After his death, a jury at Lewes Crown Court found him guilty of 10 child sex-related offences and not guilty on six counts in what is believed to be the first time a dead man has been convicted of a crime in England.

Mr Turk's family criticised the decision of the trial judge to allow jurors to return verdicts after he had died.

On Thursday, three judges in the Court of Appeal said the verdicts were wrongfully returned and must be set aside as a nullity with the convictions annulled.

Sir Brian Leveson, sitting with Mr Justice Jay and Mr Justice Garnham, said there was nothing to suggest that any of the verdicts returned were unsafe but they were, however, "irregular".

"In our judgment, there is no discretion as to the course which the judge should follow in these circumstances; he was not entitled to continue simply because he thought that obtaining verdicts from the jury justified that course."

The proceedings were brought by Mr Turk's mother Jasmine Botting, 76, from Etchingham, near Hastings, East Sussex.

During the hearing, Sir Brian said the case raised a "really important issue of criminal law".

He added: "We can recognise that for the victims, the pronouncement of the verdicts, at least in certain cases, was a vindication of their evidence."

He emphasised: "Nothing we decide in any sense should be taken as removing their appropriate feelings of vindication."

Mrs Botting has previously said: "I know for a fact that my son is innocent."

Mr Turk was a member of care staff and later head of care at the boarding school, but was not a teacher. At the time of his death, he worked as an electrician's assistant.

At an inquest in January, a coroner heard he had been prescribed anti-depressants and left suicide notes before his body was found by his stepfather.

A post-mortem examination confirmed his cause of death was by hanging. A coroner concluded he had taken his own life.

Sir Brian said that critical to the criminal justice process was that trials were neither initiated nor pursued against those who had died.

"As soon as a judge learns that a defendant has died, it is his duty to take no further step in the case against that defendant save for receiving proof of death whereupon the indictment as far as it concerns that defendant must be declared of no effect."

He added: "We appreciate that, in relation to many of the allegations pursued at trial, complainants may well consider that they had been vindicated and that, by this decision, there was important recognition of their position; so far as the record is concerned, that would be lost.

"We recognise that entirely understandable view but we repeat that it is critical to maintain an approach to criminal justice that is consistent and even-handed, maintaining a definitive position that the death of a defendant brings any criminal prosecution of that defendant to an end."

A spokeswoman for the complainants said later: "We respect the decision of the court.

"However, there should be equal regard and recognition for the feelings of those of us who were involved and who have pursued justice for our sons for many years now."

She added: "This appeal was about a point of law, not clearing Darren Turk's name."